Ozone water is known to be beneficial in many fields due to its sterilizing property, deodorizing property, effects on cells and the like. Furthermore, ozone does not affect the respiratory organs when it is dissolved in water. Accordingly, ozone water has been widely used in medical and nursing fields as well as industrial fields. However, due to the short time in which the concentration of ozone water decays, there is a strong demand for the concentration to be selected and checked on the site where ozone water is used.
One conventional ozone water concentration measurement method is iodine/pigment titration, which displays the change in color of a reagent such as potassium iodide. However, a problem with this method is that the measurement values differ between individual measurers due to the reliance on a visual determination by a measurer. Furthermore, this method requires liquid waste disposal following the measurement and there is a high cost for preparing the reagent. Furthermore, this operation is not easy, and such a complicated operation prevents the practical use thereof in typical sites where ozone water is used.
For these reasons, the method currently in use is an ultraviolet absorption method that determines the ultraviolet absorption rate of ozone water, or diaphragm-type polarography, in which electrodes and an electrolyte are shielded from ozone water, i.e. a sample solution, by a highly ozone-permeable diaphragm, and the ozone concentration is determined from the electric current when a constant voltage is applied between the electrodes, which increases proportionally to the amount of ozone that has penetrated the diaphragm and diffused in the electrolyte.
However, the problem with the ultraviolet absorption method is that it is very expensive and it is difficult to precisely measure the concentration due to ozone bubbles that scatter light transmitted from an ultraviolet absorption ozone water concentration meter. The diaphragm polarography uses a diaphragm and an electrolyte such as hydrogen peroxide, persulfic acid, a fluorine acid and a chlorine acid. Therefore, the diaphragm and the electrolyte require periodic maintenance. Further, some electrolytes may have a problem of liquid waste disposal and there is the danger that the electrolyte will corrode the electrodes.
To cope with these problems, a concentration measurement method known in the art does not use a diaphragm and also prevents the electrolyte from corroding the electrodes, in which the use of an electrically conductive diamond for a working electrode enables the working electrode to be directly dipped in ozone water, and the ozone concentration is measured from a change in the electric current value between the working electrode and a counter electrode when a changing voltage is applied between a reference electrode and the working electrode (e.g. see Patent Document 1). The working electrode described in Patent Document 1 is constituted by a silicon substrate with a thin film of boron-doped diamond formed thereon. The working electrode has a comparatively large size with an outer diameter of approximately 4 mm to 5 mm.
In a concentration measurement apparatus that uses a working electrode, a counter electrode and a reference electrode as described above, the voltage Eappl between the working electrode and the reference electrode corresponds to the electrode potential to be regulated. When a current I flows between the working electrode and the counter electrode, the voltage falls by a value IRsol (where Rsol is the effective resistance of a solution between the working electrode and a tip of the reference electrode). (This phenomenon is also referred to as IR drop.) That is, the voltage E actually applied to the electrode surface (sample solution) is decreased by the amount of voltage drop, which is represented by E=Eappl−IRsol. When the sample solution is pure ozone water that contains no electrolyte and a large electrode is used that has an outer diameter of approximately 4 mm to 5 mm like the working electrode described in Patent Document 1, the large surface area of contact with the sample solution greatly increases the solution resistance Rsol and the current I, and the resultant voltage drop becomes too large to ignore. As a result, the voltage E actually applied to the sample solution becomes much lower than the voltage Eappl between the working electrode and the reference electrode. This decreases the electric current flowing between the working electrode and the counter electrode to a level that is too low to be precisely measured. Accordingly, it is impossible to precisely measure the ozone concentration from such a low electric current.
Therefore, in the case in Patent Document 1, the use of an electrolyte is essential in order to decrease the solution resistance Rsol. However, this causes problems with the electrolyte, such as liquid waste disposal and handling difficulties.
Prior Art Document
Patent Document
Patent Document 1: JP 2007-212232A